State of the Evidence: Cannabis Use and Regulation

International Centre for Science in Drug Policy (ICSDP)August 2015

As more and more jurisdictions reconsider their cannabis policies, the public discourse is filled with conflicting evidence about the impacts of cannabis use and regulation. Cannabis causes schizophrenia. Cannabis is as addictive as heroin. Cannabis regulation leads to increased traffic fatalities. We hear claims like these all the time – but are they based on science? In our latest reports, the ICSDP investigates and provides comprehensive evaluations of the evidence for and against each claim.

Since its inception, the International Centre for Science in Drug Policy (ICSDP) has sought to ensure that policy responses to the many problems posed by illicit drugs are informed by the best available scientific evidence.

Given the robust global conversation around the regulation of recreational cannabis markets, claims about the impact of cannabis use and regulation are increasingly part of the public discourse. Unfortunately, though, these claims are often unsupported by the available scientific evidence.

Using Evidence to Talk About Cannabis is the ICSDP’s contribution to the growing global conversation on cannabis. The response guides in this report will equip readers with quick, easy, and evidence-based responses to commonly heard cannabis claims.

Using Evidence to Talk About Cannabis is comprised of two sections: Common Claims on Cannabis Use and Common Claims on Cannabis Regulation.

Common Claims on Cannabis Use presents response guides with evidence on frequently heard claims about cannabis use, including claims on the addictive potential of cannabis, cannabis as a “gateway” drug, the potency of cannabis, and the impact of cannabis use on the lungs, heart, and brain (in terms of IQ, cognitive functioning, and risk of schizophrenia).

Common Claims on Cannabis Regulation presents response guides with evidence on frequently heard claims about the impacts of cannabis regulation, including the impact of regulation on cannabis availability and use, drug crime, impaired driving, drug tourism, and “Big Marijuana.”

These response guides should be read in tandem with State of the Evidence: Cannabis Use and Regulation, a longer report that more fully details the scientific evidence on cannabis use and regulation.

Readers of these response guides will notice three repeating themes emerge through the discussion of the scientific evidence on common cannabis claims.

First, many of the claims confuse correlation and causation. Although scientific evidence may find associations between two events, this does not indicate that one necessarily caused the other. Put simply, correlation does not equal causation.

Second, for several of these claims, the inability to control for a range of variables (“confounders”) means that we often cannot conclude that a particular outcome was caused by cannabis use or regulation.

Third, many of the claims cannot be made conclusively as there is insufficient evidence to support them. This is especially true of claims related to cannabis regulation, as not enough time has passed since the regulation of recreational cannabis in Colorado, Washington State, and Uruguay to examine many of the impacts of these policy changes.

These three common pitfalls are important to take into account when reading media reports and advocacy materials that suggest scientists have conclusively made some finding related to cannabis use or regulation.

We hope that the evidence contained in these response guides meaningfully contributes to the global conversation around cannabis policy and helps policymakers, as well as general readers, separate scientific evidence from conjecture.

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